This content was first published on IGR – Italian Gemological Review no. 15 in 2022. The information provided here is therefore current as of the original publication date.
The gem market can still vividly recall the feverish rush for cobalt blue spinels from the Luc Yên district of Vietnam. As it is often the case in gemology, the market is demanding more than the vagaries of nature can provide.
Reportedly, a deposit located about 20 kilometers south-east of Mahenge, Tanzania, which for more than 20 years has been known for its red to pinkish red spinel of exceptional quality is providing some quantity of blue spinel as well. These spinel have been offered in Las Vegas show and raised a wide concern.
The new source has attracted a gem dealer based in that area in Tanzania, Wez Barber of Mahenge Gems, a company established in early 2020. He noted that miners were submitting to the company one-carat size spinels exhibiting an unprecedented blue color. In an attempt to check whether the coloration was due to cobalt, the gems were sent for identification to ICA Gemlab, GRS and SSEF.
The results from SSEF revealed that they contain cobalt as a main color bearing element (chromophore), in combination with iron. Their composition is similar to cobalt-spinel materials from other sources such as Sri Lanka, Pakistan and Vietnam.
The Swiss lab has released a short note where further comments and details are provided. These cobalt-blue spinel sourced in Mahenge, according to SSEF, present inclusions that are typical of those from Morogoro, such as oriented geometric lamellae with interference colors, along with lines of fine particles as well as colorless apatite and clusters of small zircon inclusions.
Mahenge Gems Director Wez Barber commented: «We bought as much of it as we could lay our hands on, and still are, because it was such nice material… Today there is only a fraction coming into our office compared to January of this year, but even then, back in January, there was not a lot of this material at all».
Gem News published on IGR – Italian Gemological Review #15, Autumn 2022
